This invention relates to semiconductor devices; and, more particularly, to semiconductor memory devices used in electrically alterable read only devices or ROMs, and semiconductor thin film devices or spintrons, or spin-orbitrons.
Approximately 60 years ago, the Russian scientist B. T. Kolomiyetz discovered semiconducting features in amorphous chalcogenide compounds. Kolomiyetz, using an external voltage in complex chalcogenide compounds such as AsTeJ, observed the effect of transmission from a high resistance state (OFF) to a low resistance state (ON). This effect was referred to as the effect of threshold switching.
Sometime after that, the American scientist S. R. Ovshinsky observed the effect of low-resistance state preservation after voltage removal in an amorphous chalcogenide compound GeTeSb. That effect was referred to as the effect of memory.
These discoveries served as an impetus for intensive development of a new research area; i.e., the physics of disordered chalcogenide semiconductors. Over time, switching effects were found in various complex chalcogenide compounds. However, of all the chalcogenide compounds with disordered structure having switching and memory effects, the two most distinctive compounds are SiTeAsGe (STAG) and GeTeSb (GTS). When an external voltage (U=Uth) is applied to amorphous glass Si12Te48As30Ge10, after a certain delay, one can observe an abrupt voltage drop such as is shown in FIG. 1 of the drawings. As further shown in FIG. 1, the specimen passes to an initial high resistance (OFF) state when the voltage drops to lower than Uh. The threshold switching effect on STAG compounds is both repeatable and reversible. This is why these compounds can be used as a part of electronic switches (keys).
Referring to FIG. 2, in amorphous compounds such as Ge2Te5Sb2, the ON state persists even when there is no applied voltage. To convert the material to its initial OFF state, one energizes it using a high amplitude current impulse. The ON state memory effect in the specimen is repeatable and also reversible. Accordingly, the Ge2Te5Sb2 compound can be successfully used in nonvolatile storage cell production.
Studies have shown that usage of different chalcogenide compounds leads to different results. Because of this, various models to explain the effects of memory and switching were proposed and these models are still widely used today. For instance, memory effects are connected, as a rule, to amorphous state crystallization, i.e. a thermal model; and electronic models are commonly used to explain the threshold switching effect. These models are still used despite of the fact that both the SiTeAsGe (STAG) and GeTeSb (GTS) compounds are amorphous.
Initially, the possibility of practical implementation of open switching effects in amorphous chalcogenide semiconductors universally aroused a great deal of interest. However, multiple attempts to create electronic devices on the basis of chalcogenide semiconductors have encountered many difficulties, the most important of which are their instability and unreliability. Many studies and experiments have shown that the effects of an abrupt resistance drop in amorphous chalcogenide semiconductors in a strong electric field are connected to phase transitions into a metastable state. Some examples of metastable states are described in U.S. Pat. No. 5,335,219. Issues related to the physical origin of these metastable states are still under consideration due to their complexity.
To find out more about the physical origin of these phenomena, it is first necessary to understand the characteristic features of chalcogenide semiconductors with a disordered structure. It is generally accepted that the main distinction between crystalline and non-crystalline semiconductors (those with a disordered structure and which are amorphous) is that non-crystalline semiconductors have many more structural defects than crystalline ones. In non-crystalline, amorphous chalcogenide semiconductors these defects are marked as valence-alternation pairs (V.A.P.): C3+-C1−, where C stands for a chalcogen atom. Further, the symbol at the base of the letter denotes a coordination number; that is, the number of bonds created with neighboring atoms, and the symbol at the top of the letter denotes a defect's charge.
Let me define some interesting and, to my mind, essential properties of these defects:
1. Driven by various external actions (electromagnetic field, heating, ultra-violet irradiation etc.) at these defects, a reversible redistribution of charge density occurs: C3+-C1−⇄C30. See, for example, the journal article “Threshold switching in chalcogenide-glass thin films”, published in J. Appl. Phys., vol. 51(6), p. 3289-3309, (1980), by D. Adler et. al. Redistribution of charge density occurs not only on chalcogen atoms but on other atoms included in the amorphous compound as well. This is induced by self-compensation processes, taking into account the dipole character of correlation between ions in chalcogenide compounds. The main principle of the process are described, for example, in the journal article “Self-compensation of Metastable Centers in the Chalcogenide Semiconductor Glasses”, published in Sov. Fiz. Tverd. Tela, V. 22(5), P. 785-791 (2002) by N. T. Bagraev et. al.
2. C1− is a negatively charged, singly coordinated chalcogen, or negative U-center. In this defect, two electrons are localized. The mechanism (model) of localization of two electrons was introduced by P. W. Anderson in his journal article “Model for the Electronic Structure of Amorphous Semiconductors”, Phys. Rev. Lett., V. 34, No. 15, p. 953-955, (1975). The pair of localized electrons on the negative U-center is, in essence, the analogue of Cooper's pair with small localization radius. Please refer to the Bagraev et al. article mentioned above.
Apart from the analysis of the defects features, it is also necessary to carry out an analysis of chalcogenide compounds. Percentage composition analysis of memory and switching elements on the basis of chalcogenide compounds elements demonstrates that the main chemical compound is Tellurium. Tellurium (Te) is a representative of chalcogenide material from column VI of the Periodic Table of the Elements. The structure of the valence shell of Te is 5s25p4. Te is characterized by divalent bonding and the presence of lone pair (LP) electrons, the divalent bonding leading to the formation of chain structures. Two of four p-electrons form covalent bonds with neighboring atoms. The angle between atoms in the chain is 103.2°. The van der Waal's bonds between chains is not very strong. The last lone pair of electrons takes part in creation of these bonds. The crystalline structure of Te is hexagonal and anisotropic, and a high anisotropic crystalline structure is connected to the piezoelectric properties of Tellurium crystals.
Some time ago, I worked in the Academy of Sciences of the Belarusian Soviet Socialistic Republic on various projects and programs during which I carried out many comprehensive studies on thin solid Tellurium films and its alloys. In my research I obtained unusual and unexpected results. The most interesting and essential results obtained by me and other researchers are presented below.
Te films, produced using vacuum evaporation, have many lattice defects. Depending on the texture, which is defined by the deposition conditions of Te films (rate of the deposition, temperature of the substrate etc.), these defects are connected to broken covalence bonds and Van der Waal's bonds. The breaking of both the covalent and Van der Waal's bonds create levels in a Te band-gap. See the journal article “Structural Features and Electro-conductivity of Te Thin Films”, published in Sov. Izv. Acad. Nauk, USSR, ser. “Neorg. Mater.”, V. 27, No. 9, p. 1820-1825, (1991) by B. S. Kolosnitsin, E. F. Troyan et. al. For example, impairments of Van der Waal's bonds create states at the level of the upper edge of the valence band See “Electronic structure of trigonal and amorphous Se and Te”, published in Phys. Rev., B, V. 11, No. 6, p. 2186-2199 (1975) by J. D. Joanopoulos et. al., and the candidate's thesis by E. F. Troyan on the issue in 1997. Lattice defects in Te films act as acceptors, i.e. they attract electrons both from the valence band of Te and from various impurities or additives. As a rule those chemical elements have an electronegativity less than that of Te. Hence these films exhibit p-type conduction. However, some chemical elements with an electronegativity higher than Te become acceptors at certain states. These elements include Oxygen (O) and Fluorine (F) among others.
To effectively influence electro-physical parameters, additives in thin Te films are electrically active. There are many ways to achieve an electric activity in additives (impurities) in disordered chalcogenide semiconductors. These are referred to as modification processes of chalcogenide films. A technique of modification I employ involves increasing the activity of lattice defects in Te films. In the course of interaction with these defects, the additives (impurities) become electrically active. Each modification technique is important from the point of view of achieving switching effects in thin films on the base of chalcogenide disordered compounds.
Te films produced using vacuum evaporation have a relatively high electrical conductivity (σ), as Tellurium is a semiconductor with a narrow width forbidden band (energy gap or band gap); i.e., Eg=0.335 eV. The electrical conductivity a of Tellurium films depends on the conditions of the deposition and the conductivity a is measured with an accuracy of up to 3-120 (Ωcm)−1. If measured in a vacuum, one can observe a straight dependence of current (I) from voltage (V), i.e. the IV characteristic is a linear slope. Defects will weakly influence VAC properties with the only thing that changes being a slight variation in the slope of a linear plot of VAC. No plots with a negative differential resistance (NDR) S-type at VAC have been observed.
If certain metals e.g., Aluminum (Al), Silver (Ag) or Copper (Cu)) are used as electrodes, there may be changes in the resistance of the thin film structure. This indicates migration activity of some chemical elements in a Tellurium film. It is known that ion migration in metals leads to structural rearrangements of Te films. For instance, migration of Copper ions transforms Te hexagonal structure to orthorhombic, and then to a tetragonal structure. This process of transformation is described, for example, in the journal article “Growth and Transformation of CuTe Crystals Produced by a Solid-Solid Reaction”, published in J. Non-Crystal Sol., vol. 83, p. 421-430, (1987), by S. Makoto et. al. With Silver ions the transformation is to a monoclinic structure as described, for example, in the journal article “High-resolution Electron Microscopy Observation of Solid-Solid Reaction of Tellurium Films with Silver”, published in Bull. Inst. Chem. Res., Kyoto Univ., V. 66, N. 5, p. 517-529, (1989), by S. Makoto, et. al. Tellurium is a piezoelectric material in which elastic deformation (pulsing) occurs under the influence of external electrical fields. If one supposes that, as a result of ions (impurities) migration in Te films, internal electric fields are induced, such transformations can be related to the reversed piezoelectric effects in Tellurium.
Negative Differential Resistance (NDR) plots for the deviation from linearity in observed current voltage characteristics on VAC have been found to occur when an evaporation process of Te thin films was performed in two steps with an additional operation; i.e., filling of the vacuum chamber in which process occurred with a dry Oxygen (O2) gas at a partial pressure on the order of 5.4×10−3 Pa. It is well known that Oxygen molecules adsorbed at the films' surface and during interaction with various surface states will, as a rule, create oxides. Therefore, at the beginning of the studies to which this invention is related, it was supposed that, as a result of this additional operation, the following structure was formed: M1-Te1-O(Te)—Te2-Al (where, M1-Ni, Au, Cr; O(Te)—Te oxide; Al-aluminum).
For this reason, it was decided to more carefully study physical processes occurring in multiple layer structures: M1-Te1-O(Te)—Te2-Al and compare them with their electrical properties. It will be understood by those skilled in the art that all measurements were performed in a vacuum. As shown on the right side of FIG. 3a, immediately after producing such a structure VAC characteristics analogous to VAC were measured. Over a course of 1.5-2.0 hours, the total resistance of the thin film structure increased with the VAC characteristics then becoming analogous to that of a diode. This is as shown on the right side of FIG. 3b. Referring to the right side plot of FIG. 3c, in the first and third quadrants there are plots indicating NDR.
Next, the threshold switching voltage (Uth) was gradually increased. In the 24-28 hours after the structure was produced, a stabilization of electrical properties was observed with Uth=3.5-5V, with a high-resistivity for ROFF and a low-resistivity RON; i.e., ROFF/RON being on the order of 102-103. It was further found that if Tellurium film was evaporated at a rate of deposition of V2-10.0 nm/s, then the thin film structures switched as memory elements with Uth.1=4.5-5V and ROFF/RON=103 (see FIG. 2), and at V1=2.0 nm/s, as threshold switching elements with Uth.=3.5-4V and ROFF/RON102-103 (see FIG. 1). When tellurium thin films were evaporated at a rate of deposition of V3=5.0-6.0 nm/s, the inventor saw both memory effect and threshold switching effect in the same cell at the same time. It is particularly pointed out that at the same cell, the main distinctive feature of the effects observed is a significant difference in magnitude RON of these states: i.e., RON of the cell, which is in the threshold switching state, is significantly higher than RON in memory cell.
Electrical reproduction of memory elements from a low resistance ON to a high resistance OFF state was achieved only with a change in polarity of the voltage applied to the electrodes. Threshold switching elements independently switched to the OFF state when the voltage on the electrodes was less than Uh (Again see FIG. 1). And, memory elements were found to be very unstable in the ON state. It will be noted that both memory and threshold switching elements were defined and influenced by the threshold currents Ith (see FIGS. 1 and 2) that define the temporal stability of the main electrical parameters including the number (Nswitch) of switching cycles of the thin films structures. The lower the value of Ith., the more stable and long lasting was the element. A characteristic feature of the structures produced was that they switched from the OFF to ON state only when there was a negative potential on the Aluminum electrode. Attempting to switch from the OFF to ON state, by supplying a positive potential on the Aluminum electrode, produced thin film structures at U>9-10V inevitably resulted in a break down.
Direct correlation between electrical parameters of the produced thin film structures and the polarity of the applied voltage places in question widely used switching effects models in chalcogenide semiconductors. Moreover, in production of memory and threshold switching cells, the same chemical elements were used. Only one technological parameter was changed; i.e., the deposition rate of Te thin films, and this resulted in different observed effects. For example, during production of memory cells, more high deposition rates were used; while, during production of threshold switching cells low deposition rates were used. Research on the structure of Te films has shown that in Te films produced at high deposition rates (V2≧8.0-10.0 nm/c and d≧50 nm) defects of covalence bonds prevail. On the contrary, in films deposited at low deposition rates (V1=2.0-3.0 nm/c and d≦50 nm), defects in Van der Waal's bonds prevail.
Based upon the large amount of data obtained and results achieved in the course of studying these structures, a new model was worked out. The main ideas of this new model were introduced and implemented by the inventor in 1996. Since then, the inventor has been continuously working on his theory and has been able to update his model. Using data obtained after implementation of his new model in the production of switching elements has allowed the inventor to achieve new results that he forecasted earlier. The inventor also discovered that results obtained with his improved model can also be implemented in other applications.